Digital watermarking is a technique that comprises adding information into a digital audio, picture or video signal in such a manner that the added information is difficult to remove and may be used to identify, prevent, or discourage unauthorized copying of digital media. A watermarking technology described in U.S. Pat. No. 7,058,809, entitled “Method and system to uniquely associate multicast content with each of multiple recipients,” description and drawings of which is incorporated herein by reference in its entirety, is based on the concept of generating, at the head-end for a digital TV broadcast system, two or more differently watermarked instances for the same content element and repeating the procedure for a sequence of content elements. The content stream broadcast to recipient devices then includes differently watermarked copies for the sequence of the content elements. An example of a part of a content stream including such watermarking is shown in FIG. 1 as a content stream 100. The content stream 100 illustrates a rendering order of content elements. As explained in greater detail below, a rendering order of the content elements could be different from the order in which the content elements are broadcast by the head-end and could be different from the order in which the content elements are received and/or decoded by a recipient device.
As shown in FIG. 1, the content stream 100 includes content elements C1-C7. Two copies of each of the content elements C3, C4, and C5 are watermarked with two different watermarks. Thus, a first copy of the content element C3 is watermarked with a watermark WS3,0, a second copy of the content element C3 is watermarked with a watermark WS3,1, a first copy of the content element C4 is watermarked with a watermark WS4,0, a second copy of the content element C4 is watermarked with a watermark WS4,1, a first copy of the content element C5 is watermarked with a watermark WS5,0, and a second copy of the content element C5 is watermarked with a watermark WS5,1.
All of the content elements shown in the content stream 100 are broadcast by the head-end to each of the recipient devices, although not necessarily in the same order as shown in FIG. 1. In addition to receiving the broadcast content, a particular recipient device is configured to receive (e.g. from the head-end, from the smart card within the recipient device or from some other source) selection information that provides instructions for the recipient device as to which single copy of the multiple copies of the watermarked content elements the device should select. After the recipient device selects the appropriate copies of the watermarked content elements according to the received selection information, the decoded output at the recipient device will comprise a unique sequence of watermarks. Having a unique sequence of watermarks at the decoded output content of each recipient device allows e.g. identification of source of illegal copies of the content.